The present invention relates to a light modulator, a projection display apparatus and an image projection method that achieve higher image quality for images projected onto a screen while restrict flickers that occur due to asymmetric waveforms of AC voltages applied to electrodes that face each other in a liquid crystal display device.
Projection display apparatuses have a mechanism in which light is emitted from a light source to a liquid crystal display device that forms an optical image corresponding to an input video signal, which is then projected onto a screen.
One factor that affects the quality of images projected onto a screen in projection display apparatuses is the electro-optical conversion performance of a liquid crystal display device driven by input video signals.
A liquid crystal display device is equipped with: a common electrode; a large number of pixel electrodes arranged as facing the common electrode and each being controlled by an associated drive transistor; and a liquid crystal filled between the common electrode and each pixel electrode. A voltage to be applied across the common electrode and each pixel electrode is an AC voltage that is reversed in polarity per frame or line period.
The AC voltage waveform, however, suffers asymmetric deformation between the positive and negative polarities when applied across the common electrode and each pixel electrode. The following are several factors related to the asymmetric deformation: variation in the characteristics of drive transistors used in driving pixel electrodes; variation in the electro-optical characteristics of liquid crystals; variation in the condition of liquid crystals in alignment; and contamination of impurities to liquid crystals; and difference in materials between the common electrode and the pixel electrodes that face each other.
Such an asymmetric-waveform AC voltage causes a common-electrode voltage to be shifted from the center voltage of the AC voltage that is being alternately reversed in polarity with respect to the center voltage. The common-electrode voltage (referred to as Vcom, hereinafter) is a DC voltage to be applied to the common electrode. The shifted DC voltage is then supplied to the liquid crystal display device, thus resulting in flickers occurring on projected images.
When flickers occur on projected images, they cause the images to suffer higher black levels which lead to lower contrast and narrower display gradation. Moreover, while flickers are occurring, DC voltages being supplied to the liquid crystal display device cause ionic substances involved in liquid crystals to be pulled to either of the common or pixel electrode. Such a phenomenon leads to burning on the liquid crystal display device, resulting in lower operational reliability of the display device.
In order to overcome such problems, the US patent application publication No. 2006/0231794 A1 (referred to as Citation 1, hereinafter) discloses a technique to restrict occurrence of flickers with adjustments to the voltage Vcom (referred to as V com-adjustments, hereinafter) to be applied to the common electrode.
A projection display apparatus disclosed in Citation 1 is equipped with: a light source; liquid crystal display devices for forming an optical image in accordance with an input video signal when light is emitted thererto from the light source; an optical system with a projection lens for enlarging and projecting the optical image onto a screen (each liquid crystal display device being installed in the optical system); an optical sensor for detecting the intensity of the light after emitted to each liquid crystal display device; a drive circuit for driving each display device; and a control circuit for controlling the drive circuit based on the detected intensity.
Such an optical sensor is used for detecting flickers, or an unsteady light that goes on and off quickly. In Citation 1, the optical sensor is provided at an outer edge of the screen or in the optical system.
The projection display apparatus disclosed in Citation 1 displays optical images on a screen, as explained below.
When light is generated from the light source and emitted to each liquid crystal display device, an optical image is formed at each display device in accordance with an input video signal. The optical image is supplied to the optical system through which the image is enlarged and projected onto a screen provided at a specific location including a focal point.
When Vcom-adjustments start to restrict flickers, a raster test pattern signal pre-stored in the drive circuit is supplied to each liquid crystal display device which forms a raster pattern image. The raster pattern image is then projected onto the screen via the projection lens of the optical system.
The variation in intensity of light that carries the raster pattern image is detected by the optical sensor provided at the outer edge of the screen or in the optical system. The detected variation in intensity indicates flicker leveles on the projected raster pattern image, which enables calculation of a Vcom level to be applied to the common electrode of each liquid crystal display device at which level flickers are restricted. The voltage Vcom is then adjusted to the calculated Vcom level by the control circuit so that flickers cannot be observed on the projected images.
However, the known projection display apparatus of Citation 1 has the following disadvantages due to the optical sensor provided at an outer edge of the screen or in the optical system.
When the optical sensor is provided at an outer edge of the screen, it cannot detect enough optical intensity for Vcom-adjustments because the outer edge of the screen is always at a black level during image projection. Higher accuracy of the Vcom-adjustments requires several optical sensors to be provided at outer edges of the screen with integration of signals detected by the optical sensors to give larger amplitude to the detected signals or a longer measuring period at the optical sensors to gain higher accumulated optical intensity. The arrangements of several optical sensors, however, require wider space at the outer edges of the screen and the integration of detected signals causes a longer Vcom-adjustment period. A longer measuring period also leads to a longer Vcom-adjustment period.
Moreover, when the known projection display apparatus is reinstalled in another place, the optical sensor provided at the outer edge of the screen is also required to be reinstalled at other parts of the outer edges of the screen. Thus, reinstallation cannot be done easily.
When the optical sensor is installed in the optical system of the known projection display apparatus, it must be located on the optical path from the liquid crystal display device in the optical system to the screen, which may result in the shade of the optical sensor being projected onto the screen.
Moreover, installation of the optical sensor in the optical system causes a higher cost for the optical system. Because the installed sensor requires reinstallation when the projection lens is replaced with another one depending on the installation space for the known projection display apparatus.
Furthermore, the known projection display apparatus has to perform the Vcom-adjustments with sequential irradiation of R-, G- and B-rays onto the optical sensor if only one sensor is installed or with irradiation of white light if the optical sensors for the colors of R, G and B are installed. The single optical sensor requires three times of irradiation of R-, G- and B-rays, which results in a longer Vcom-adjustment period. Installation of the optical sensors for the colors of R, G and B causes increase in the number of sensors and the installation space, which results in difficulty in reinstallation of the optical sensors when the projection display apparatus is moved to another place.
Furthermore, the known projection display apparatus requires the optical sensor(s) to be located in the optical path as being apart as much as possible from the center of the optical path in the optical system so that no adverse effects of the shade of optical sensor(s) are given to the projected images. Thus, the Vcom-adjustments must be performed with measurements of optical intensity at an outer edge of a projected image and hence cannot restrict flickers over the entire projected image when flickers irregularly occur over the display area of each liquid crystal display device.
Moreover, flickers that occur on projected images involve a flicker so-called a lamp flicker of light discharged from the light source depending on the temperature of the electrode of the light source and the surface conditions of the electrode, in addition to an unadjusted Vcom level. The known projection display apparatus inevitably performs excessive Vcom-adjustments to the flickers involving the lamp flicker, which may result in failure of the Vcom-adjustments, or incomplete flicker elimination.